Swords Without Master blew the doors off for me in many ways, but I think the idea of specifically structuring different parts of the game to achieve different goals in the fiction, but all within a cohesive structure and generating a continuous narrative, was the biggest. It’s really quite different from games that have lots of rules for different things you do as character actions (like say 90s Storyteller System or GURPS) and unified conflict resolution games (most of my stuff! But say Primetime Adventures as an ur-example) and one core set of mechanics with variations based on character abilities (Apocalypse World, most D&D when you get right down to it).

The rules are player-facing instead of character-facing, but the game still deeply cares about the characters and really only works when you play the characters hard, and that’s something that I probably couldn’t even have conceived of on my own.